Many types of integrated circuits are fabricated using layers of conductive, semiconductive, and/or insulating materials. For example, an integrated circuit may include a substrate in which a number of active devices (such as transistors) are formed. Such active devices are then connected to one another by one or more conductive or semiconductive layers (referred to herein as “conducting layers”). The interconnecting conducting layers are separated from one another by insulating layers. Insulating and conducting layers are typically deposited according to a predetermined deposition “recipe” which may define the various materials, conditions and environment used to deposit a layer.
An insulating layer may perform a variety of functions in an integrated circuit. For example, an insulating layer may serve to electrically isolate one conducting layer or structure from another. Such isolation includes both vertical isolation, typically between layers, and horizontal isolation, typically between conductive structures, such as contact structures.
A typical contact structure may include a contact and/or via is formed by etching a hole through one or more insulating layers, and then filling the hole with a conductive or semiconductive material. As integrated circuit density increases, contact structures may be situated closer to one another. Consequently, there is a need to form contact holes with controllable features.
A contact hole feature may depend upon the insulating material in which the contact hole is formed. An insulating material with desirable features may include a doped silicon oxide layer formed with a high density plasma. One approach to forming contact holes through a high density plasma insulating film is shown in commonly-owned, co-pending patent application Ser. No. 09/300,817, titled “Methods of Filling Constrained Spaces with Insulating Materials and/or Forming Contact Holes and/or Contacts in an Integrated Circuit” by Jengyi Yu (referred to herein as Yu). The contents of the patent application are incorporated by reference herein.
Doped silicon oxide films may provide advantageous step coverage and reflow properties over other insulating materials, including undoped silicon oxide. In many processes, a doped silicon oxide may be formed with a dopant source and a base material source. A dopant source may provide a dopant, such as boron or phosphorous, while a base material source may provide one or more of the elements of the material that is to be doped. In the case of silicon oxide, a base material source may provide silicon.
One concern with the formation of doped silicon oxide films can be the uniformity of doping within a layer. Lack of uniformity in doping may adversely affect other process steps. For example, for many silicon oxide etches, the rate at which silicon oxide is etched may vary according to doping concentration. In general, the higher the doping concentration, the higher the etch rate. Thus, if a contact hole is formed through an insulating layer having non-uniform doping, the higher doped regions may etch at a faster rate than lower doped regions. This may result in contact holes with walls having undercut regions. One particular example of such undercutting will now be described.
Referring now to FIG. 1, a response 100 is shown illustrating the doping concentration of a layer of phosphosilicate glass (PSG) formed a constant flow rate of source gases. More particularly, the example may result from a flow ratio of 40.5%, where the flow ratio is the ratio between a dopant source flow rate (e.g., phosphine PH3) and base material source flow rate (silane SiH4). Data values for a response 100 may be generated by a secondary mass ion spectrometry (SIMS) profile. As response 100 shows, a PSG layer may have an essentially uniform doping from a top surface of a PSG layer (0 μm depth) to a depth of about 0.4 μm. However, from a depth greater than 0.4 μm to a substrate surface (0.6 μm depth), a PSG doping concentration may increase.
It is understood that the graph of FIG. 1 may represent a dopant concentration as portions of a PSG layer are removed, beginning at a top surface. Thus, a substrate is understood to exist at a depth of about 0.6 microns. An initial 0.2 micron thickness of the PSG layer is at a depth of about 0.6 microns to 0.4 microns. An initial 0.4 microns thickness is understood to be at a depth of about 0.6 microns to 0.2 microns.
Thus, as shown in response 100 of FIG. 1, despite a uniform source gas ratio, non-uniformity can result. It is believed that such non-uniformity in a PSG film can be related to variations in substrate temperature. As but one example, FIG. 2 shows a response 200 illustrating a substrate temperature during the deposition of a high density plasma (HDP) PSG film. As shown in response 200, prior to deposition (time 0-100), a substrate may have a certain temperature (330° C.). However, as the deposition process proceeds, a substrate temperature may rise.
As shown in FIG. 3, for a given source gas flow rate, variations in temperature can result in variations in doping concentration. More particularly, FIG. 3 includes a response 300 that shows how doping concentration may go down as temperature increases.
As noted above, a higher doping concentration in one portion of an insulating layer may result in undercutting in a wall of contact hole. FIG. 4 shows a side cross sectional view of a contact formed in an insulating layer having a doping concentration gradient. More particularly,
FIG. 4 shows a substrate 400 on which an insulating layer 402 is formed. An insulating layer 402 may have a doping concentration with a gradient. The gradient may include a doping concentration that increases near the surface of a substrate 400. Such a gradient may be represented by response 100 in FIG. 1.
A contact hole 404 may be etched through an insulating layer 402 having a non-uniform doping. As also noted above, regions having a higher level of doping may etch at a faster rate than regions of lower doping, especially during wet chemical processing following a contact hole formation. Consequently, a contact hole 404 may include an undercut formation 406 (“foot”). An undercut formation 406 may complicate subsequent manufacturing of an integrated circuit and/or lead to defects. As but one example, if a contact hole 404 is filled with a conductive material, and undercutting is severe enough, such a material may form a short circuit with an adjacent conductive structure.
In light of the conventional approach set forth above, it would be desirable to arrive at some way of forming an insulating material having a uniform doping concentration on a substrate with a varying temperature.